This proposal aims to develop a structurally based model of carotenoid participation in light harvesting and photoprotection in vivo. He isolates, purifies, reconstitutes, exchanges pigments in, crystallizes and genetically modifies bacterial reaction center and antenna complexes. He uses tools such as X-ray diffraction, NMR, and Raman spectroscopy to examine structures directly and also employs EPR and optical spectroscopy. The aim is to determine the structure of carotenoids as they occur in these photosynthetic units, to generate a complete understanding of the relationship between the spectroscopic signatures and structures of the carotenoids and to discover the relations between these structures and the specific roles of the carotenoids in light harvesting and photoprotection. The data are expected to provide a basis from which to understand more complex systems; e.g. higher plant pigment-protein complexes whose properties are more complex than the bacterial systems. The specific aims of the proposal are: (1) To examine the structures of carotenoids in photosynthetic pigment protein complexes; (2) To explore the relationships between the carotenoid structures and their spectroscopic properties; (3) To determine the relationships between the carotenoid structures and their mechanisms of light-harvesting and photoprotection.